FIG. 3 shows an example of the prior art two-port circuit network measurement devices. This arrangement is generally called an "S parameter test system," "network analyzer system," or "impedance analyzer." These systems consist of a network analyzer 1 and a lower test set 2. However, there are also devices in which both parts are combined into one. Here, we shall take the general separate-type S parameter test system as an example.
The test set is provided with a number of bridges equal to the number of measurement ports, which conduct the measurement signals from the network analyzer to the circuit being measured and separate the transmitted and the reflected signals necessary to measure the S parameters. That is, in the case of two-ports, 2 sets of bridges are provided. The reception of the measurement signals between the network analyzer and the test set is performed through four coaxial cables 3. In addition, although they are not shown in the figure, suitable cables also generally connect the network analyzer and the test set at their rear panels, for power supply access and to exchange control signals. Moreover, there are also cases in which the circuit network measurement device is controlled by a computer (although this is also not shown in the figure).
Two connectors 5, serving as measurement ports, are provided on test set 2. Ordinarily, the types of these connectors are N, BNC, or APC-7, etc., coaxial connectors. When a circuit network being measured is connected directly to these two connectors 5, the connectors become reference planes, i.e., the measurement ports. However, the circuit network to be measured is rarely connected directly to connectors 5; generally, coaxial cables 6 are connected to connectors 5, and the circuit network to be measured is connected to connectors 7 at the front ends of coaxial cables 6. At such time, connectors 7 at the ends of cables 6 become the measurement ports (reference planes of the measurements). In FIG. 3, the connectors on the test set side of coaxial cables 6 are omitted from the diagram.
Among the methods for calibrating the circuit network measurement device, the full two-port calibration method is known as the best method. The full two-port calibration method consists of (a) one-port calibration, (b) isolation calibration, and (c) through calibration. In one-port calibration, three known impedances are prepared as standards; the port is calibrated by connecting these standards successively to one port. The same calibration is also performed on the other port.
Ordinarily, the three known impedances used are open, short, and load. In isolation calibration, each port is terminated and the isolation between the ports, i.e., leakage signal, is measured. In through calibration, the ports are connected directly to each other, and the transmission properties are measured. The errors in these three measurements are calculated, and are corrected. Details of the full two-port calibration method are given in the following reference: "Accuracy Enhancement Fundamentals--Characterizing Microwave Systematic. Errors," HP 8753C Network Analyzer Operating Manual, Reference Section, Appendix to Chapter 5.
The case in which there are two measurement ports was discussed above. In complex circuit networks that are to be measured, two measurement ports are insufficient, and many measurement ports are necessary. The method of calibration is the same for the case in which many measurement ports are provided. As shown below, however, when the number of measurement ports becomes large, the number of combinations of measurement ports becomes large, and the calibration standards and cables, etc., must be reconnected many times. As a result, not only the work and time involved become problems, but abrasion and damage to the connection planes of the connectors and connection mistakes, etc., become problems.
Here, the number of times the standards and cables are connected and disconnected shall be considered when a circuit network measurement device with n measurement ports is calibrated by the full two-port method. Cables 6 are connected to test set 2. This connection is not counted as the number of connections and disconnections, since it is not related to calibration. In one-port calibration, three standards are connected and disconnected per measurement port, so that the number of connections and disconnections is 3 n. In isolation calibration, if it is assumed that n terminal resistances are prepared and that the measurements are performed after all the ports are terminated, there are n connections and disconnections. However, the number of measurements is the number of combinations of two-ports selected from n ports, or nC2. In through calibration, the number is the number of combinations of two-ports selected from the measurement ports of the ends of n cables, so that it is either nC2 (when the connectors can be connected to each other) or nC2+1 (when intermediate adapters are necessary for the connection).
From these facts, it can be seen that when n is six or less, the number of connections and disconnections for one-port calibration is greater than for others, and when n is seven or greater, the number of connections and disconnections for through calibration is the most.